As the size of electromechanical, electro-optical, and electronic systems shrink to micrometer and nanometer scales, components within those systems necessarily shrink as well. Smaller components require more precise processing techniques to ensure optimal system performance. Imperfections in individual components of a particular system can affect the macroscopic performance of the system and lead to failure of other components, loss of sensitivity, or loss of accuracy. Improving the operational lifetime of a particular component can be achieved by making the component and the component's interaction with the system more robust. Reducing loss of system sensitivity or loss of system accuracy can be achieved by reducing imperfections in individual components.
For example, in a micro-fabricated device employing a membrane or plate structure (collectively “membrane”) suspended over a cavity, the boundary condition at the interface between the cavity and the membrane determines the robustness or lifetime of the membrane. An uneven or jagged boundary condition causes a stress concentration that can ultimately cause the membrane to fracture and fail at that boundary. When the system is exposed to or operates in a fluidic environment, the failure of the membrane can cause leakage into other components in the system. Leakage can lead to costly contamination or damage to the entire system.
A poorly designed and/or poorly fabricated boundary condition between the cavity and the membrane of an acoustic device can produce a device having a disadvantageous frequency response (e.g., low Q, low signal-to-noise ratio, or a high modal overlap and spillover). A disadvantageous frequency response is one where it is difficult to distinguish between frequency modes of the device. A disadvantageous frequency response also affects the measurement capabilities of the device by reducing the value of quality (“Q factor” or “Q”) or the loss of the system. The Q of a system generally compares the time constant for decay of an oscillating physical system's amplitude to the oscillation period. Alternatively, Q compares the frequency at which a system oscillates to the rate at which the system dissipates energy. In some cases, Q is defined as the ratio between the resonant frequency of a system and the bandwidth of frequencies (Δf) over which the energy in the system is greater than half the peak value.
A subset of acoustic devices is known as resonant devices. Resonant devices have one or more resonant frequencies. The resonant frequencies of resonant devices depend on the Q factor of the resonant device. In some embodiments, standing waves associated with operation of the membrane of the resonant device are used for sensing and actuating purposes. Acoustic devices also include a family of resonant devices known as flexural plate wave (“FPW”) devices. The problems discussed above also occur in FPW devices.
Additionally, imperfections in the cavity walls as a result of limitations of fabrication methods can affect the performance of the system. For example, when a cavity wall has a rough surface, introduction of a fluid to the cavity can erode the wall and cause portions of the wall to break or flake off and accumulate on the membrane as debris. The debris supplies a load on the membrane. In cases where the membrane is a sensor, the debris on the membrane can potentially result in a spurious signal and can affect the sensitivity of the device. In the case of a sensor or actuator, debris on the membrane can interfere with or change the interaction of the device with the fluid.
Furthermore, a rough cavity wall can lead to incomplete wetting of the wall, which can lead to formation of bubbles of trapped gas along the wall. A relatively rough cavity wall provides multiple sites for bubble formation or gas nucleation. As fluid flows over the cavity wall, bubbles can dislodge from the nucleation sites and move toward the membrane. The interaction of bubbles with the membrane affects the membrane's interaction with the fluid, which affects the performance of the membrane as an actuator or sensor.